Gas discharge lamps have to be ignited by means of a high voltage. After ignition, the lamp warms up from a starting temperature to an operating temperature during a warm-up phase. The voltage present across the gas discharge lamp after the ignition is termed the arc voltage and is substantially independent of the lamp current within wide limits. The arc voltage rises during the warm-up phase from a starting arc voltage up to an operating arc voltage. The warm-up phase is followed by an operating phase in properly functioning gas discharge lamps.
A distinction is made between high-pressure and low-pressure gas discharge lamps in lamp technology. With high-pressure gas discharge lamps, also termed lamps below, it is essential for the mode of operation that the pressure in the lamp vessel rises during the warm-up phase from an initial pressure up to an operating pressure. This is one reason why the invention described below can be used with particular advantage with high-pressure gas discharge lamps. However, it can also be used with low-pressure gas discharge lamps.
It is customary during the operating phase for the operating device to regulate the power of the lamp to a desired power. Since the arc voltage is low during the warm-up phase, in the case of pure power regulation a high lamp current would be required during the warm-up phase in order to set the desired power. This current can be higher by a multiple than the lamp current during the operating phase. This would lead to a destruction of the lamp electrodes. Consequently, in the prior art the current supplied to the lamp by the operating device during the warm-up phase is limited to a constant warm-up current. The lamp is therefore fed with the constant warm-up current at least during a first segment of the warm-up phase. The arc voltage rises in the course of the warm-up phase. Once the arc voltage has reached a value that yields the desired power in conjunction with the constant current, the power regulation begins to operate. As the arc voltage rises further, the lamp current is reduced so far by the power regulation that the desired power is established. The warm-up phase is concluded when the arc voltage has reached the value of the operating arc voltage. The operating arc voltage exhibits manufacturing tolerances and also changes during the service life of a lamp. The operating arc voltage is therefore defined by the arc voltage that remains substantially constant during a time range given the desired power. The time range to be considered is one minute, for example. Correlated with the operating arc voltage is an operating lamp current that yields the desired power together with the operating arc voltage.
The following is to be noted for the value of the warm-up current: during the warm-up phase, it is necessary to couple so much power into the lamp that the pressure in the lamp, and thus the lamp voltage, rises steadily until the operating arc voltage is reached. Otherwise, the case can arise that the lamp persists in a stable state during the warm-up phase, and the desired power is not reached. In order to exclude this case reliably, a warm-up current is selected in the prior art that is substantially above the operating lamp current. This is described in document U.S. Pat. No. 5,083,065 (Sakata). One aspect in the selection of the warm-up current is also the desire for the shortest possible warm-up phase in order to reach a desired luminous flux in the shortest possible time.
However, a high warm-up current constitutes a heavy load on the electrodes, and this causes damage to the electrodes and therefore reduces the lamp service life. The electrodes are damaged either by overheating, which causes fusing and erosion, or by so-called sputtering, which is caused by ions that strike an electrode at high speed.